The present application relates to the reduction of undesired amplitude modulation, and more particularly, but not exclusively, relates to the reduction of residual amplitude modulation in frequency-modulated signals carrying information.
Various techniques for detecting extremely low concentrations of a substance of interest have been developed that involve Frequency Modulation (FM) of a laser beam. These techniques include Frequency Modulation Spectroscopy (FMS), wideband FM, Wavelength Modulation Spectroscopy (WMS), and the like. Generally, the frequency-modulated laser light is directed through an analyte that is characterized by spectral absorption and/or dispersion of the interrogating light. The returned light, an altered form of the interrogating light, is detected and evaluated to determine these spectroscopic characteristics of interest. More particularly, FMS can utilize a one-tone modulation technique or a two-tone modulation technique as is further explained in the article by Silver, Joel A., Frequency-Modulation Spectroscopy for Trace Species Detection: Theory and Comparison Among Experimental Methods, APPLIED OPTICS, Vol. 31, No. 6 (20 Feb. 1992), which is hereby incorporated by reference.
Frequency modulation of laser light typically results in an undesired amount of Amplitude Modulation (AM), so-called Residual Amplitude Modulation (RAM), due to nonideal behavior of the laser and/or other elements of the system. Unfortunately, residual amplitude modulation limits the sensitivity of FM techniques with lasers—functioning as a form of noise that can at least partially obscure spectroscopic information in the output signal.
One scheme to reduce residual amplitude modulation depends on frequency modulation of the laser beam with an Electro-Optic Modulator (EOM). Unfortunately, the frequency modulation index range available with existing EOMs is somewhat limited—such that very high modulation frequencies are needed—correspondingly increasing cost and complexity of the system. Furthermore, suitable EOMs are not available for certain interrogation wavelength ranges that have promising applications.
In principle, it has been recognized that residual amplitude modulation can be avoided by detecting an absorption signal in a phase-sensitive manner when the phase of the residual amplitude modulation is different from the phase of the frequency modulation signal. Generally, it is optimal that this phase difference be 90°. However, this scheme is also limited by very stringent requirements regarding linearity and dynamic range of various system elements. Furthermore, phase of the residual amplitude modulation needs to remain stable for such schemes to be effective.
Still another scheme exists peculiar to lead-salt lasers. For this scheme, the laser is operated at or near an operational limit for which laser output power is generally independent of electric current. Drawbacks of this approach include a limited availability of wavelengths for absorption detection and the adverse impact such operation has on the lifetime of the laser.
Accordingly, there is a need for further contributions in this area of technology.